This invention relates to vulcanizable fluoroelastomer compositions exceptionally well suited for molding intricately shaped articles and readily recyclable even after press-curing.
Many fluoroelastomers have achieved commercial success in such applications as, for example, gaskets, seals, diaphragms, and tubing, where high temperature resistance and chemical inertness are required. Such articles are formed by various techniques well known in the polymer fabrication art, including extrusion, injection molding, and transfer molding. The elastomer composition must be fluid enough at the operating temperature to flow evenly into the mold or through a die, but it must be sufficiently cured by the time it leaves the forming equipment to maintain its shape during subsequent handling, including actual demolding. It also must be sufficiently strong to withstand demolding without tearing. If the degree of cure in the forming equipment (sometimes referred to as the "press-cure") is high, sufficient flow may not be achieved, and the fabricated articles are imperfect. This is particularly true for short press-cure cycles.
Another problem associated with a high press-cure is the difficulty of recycling the flashings and scrap elastomer. While a fair proportion of fabricated articles will be imperfect, they cannot be readily recycled, thus adding to the cost of the commercially acceptable articles produced in those operations.
Vinylidene fluoride copolymers with other ethylenically unsaturated fluorinated monomers can be cured by crosslinking with aromatic polyhydroxy compounds in the presence of accelerators and of divalent metal oxides or hydroxides. The accelerators usually are quaternary phosphonium, arsonium, or stibonium compounds, as described in U.S. Pat. No. 3,876,654 to Pattison, but can also be quaternary ammonium compounds, as shown by U.S. Pat. No. 3,988,502 to Patel et al; substituted guanidines or amidines recited in U.S. Pat. No. 3,686,143 to Bowman; or cyclic polyethers taught in U.S. Pat. No. 3,580,889 to Barney et al. The teachings of those patents are incorporated herein by reference.
It is highly desirable to provide an improved process and compositions which would produce a low but satisfactory degree of cure in the press cycle.